In triangle $ABC$, $AB = 7$, $AC = 15$, and the length of median $AM$ is 10.  Find the area of triangle $ABC$.
Solution: Extend $AM$ to $D$ so that $MD = MA$.  Then triangles $AMB$ and $DMC$ are congruent, so triangles $ABC$ and $ACD$ have equal area.

[asy]
unitsize(0.3 cm);

pair A, B, C, D, M;

A = (-7/sqrt(37),42/sqrt(37));
B = (0,0);
C = (2*sqrt(37),0);
M = (B + C)/2;
D = 2*M - A;

draw(A--B--C--cycle);
draw(A--D--C);

label("$A$", A, dir(90));
label("$B$", B, SW);
label("$C$", C, NE);
label("$D$", D, S);
label("$M$", M, SW);

label("$7$", (A + B)/2, W);
label("$15$", (A + C)/2, NE);
label("$10$", (A + M)/2, SW);
label("$10$", (D + M)/2, SW);
label("$7$", (C + D)/2, E);
[/asy]

The semi-perimeter of triangle $ACD$ is $(7 + 15 + 20)/2 = 21$, so by Heron's formula, the area of triangle $ACD$ is $$\sqrt{21 (21 - 7)(21 - 15)(21 - 20)} = \boxed{42}.$$